A wide range of implantable medical devices are provided for surgical implantation into humans or animals. One common example is the cardiac pacemaker. Another is the implantable cardioverter defibrillator (ICD). Other examples include devices for stimulating or sensing portions of the brain, spinal cord, muscles, bones, nerves, glands or other body organs or tissues.
Many such implantable medical devices include one or more electrical leads for conducting sensed electrical signals away from a particular part of the body such as the heart or for conducting stimulating electrical signals to the particular part of the body. In the case of a pacemaker, the sensed electrical signals are typically representative of P-waves and R-waves. The stimulating electrical signals are small pulses of electricity, on the order of micro-joules, for stimulating the heart in the event that the expected P-waves or R-waves are not detected. In the case of an ICD, the stimulating signals are typically 10 to 30 joule pulses of electricity provided to terminate tachycardia or fibrillation.
The electrical leads, for various reasons, may cease to function properly. For example, the electrical lead may suffer some minor damage during implantation that may affect the electrical insulation of the lead. This type of damage may not be initially detectable but may manifest itself after an extended period of time. In particular, stress imposed on the electrical lead as a result of the normal movements of the body may further damage the lead resulting in a complete or otherwise significant breakdown in the electrical insulation of the lead. In other cases, the lead itself may fracture. If either type of damage occurs, serious or even disastrous consequences may result. For example, in the case of a pacemaker or ICD electrical stimulation signals intended for the heart may be shunted to other parts of the body rendering the stimulation signals ineffective for pacing the heart or for terminating tachycardia or fibrillation.
Accordingly, various techniques have been developed for testing implanted electrical leads to detect lead faults such as lead fractures or complete breakdowns in insulation. To this end, many pacemakers now include circuitry for periodically or continuously testing the impedance of electrical leads connected to the pacemaker and any significant deviation from a range of acceptable impedance values is recorded within the pacemaker (subject to memory-space limitations) for subsequent downloading to an external monitoring device such as a pacemaker programmer. The downloaded data is analyzed by the external monitoring device to determine if an unexpected impedance value had been recorded by the pacemaker. Typically, the external monitoring device determines whether any of the recorded impedance values exceeds a predetermined upper threshold, such as 2000 ohms, or any falls below a predetermined lower threshold, such as 200 ohms. If the lead impedance exceeds the upper threshold, the lead is presumed to have fractured and must be replaced. If the impedance falls below the lower threshold, the insulation of the lead is presumed to have failed and the lead therefore also must be replaced. In either case, an audible warning signal or other simple notification is provided to the physician operating the external monitoring device. In other cases, the testing of the lead occurs while the pacemaker is in communication with the external monitoring device allowing such warning signals to be generated immediately. In still other cases, the pacemaker itself tests for unexpected impedance values and generates a warning signal within the patient by producing a high-pitched audible tone or by providing a mild, but noticeable electrical shock, to the patient.
Although the testing of electrical leads and the generation of simple warning signals upon the detection of an unexpected impedance value represents an improvement over systems which do not provide for lead testing, considerable room for improvement remains. In particular, with the generation of only a simple warning signal, the physician may not be provided with sufficient information to readily determine the exact nature and seriousness of the lead fault. The physician may not be able to determine easily, for example, whether the fault is a permanent fault or merely an intermittent one. If intermittent, the physician may not be able to determine easily whether the intermittent fault lasts only momentarily or for a longer period of time. As can be appreciated, further information regarding the exact nature of the lead fault may be required by the physician before he or she can determine the seriousness of the fault and, in particular, determine whether the lead must be replaced immediately or whether such action can be deferred at least temporarily. Moreover, further information regarding the exact nature of the fault may even be required before the physician can properly diagnose the patient. For example, a patient exhibiting an intermittent arrhythmia may have a faulty lead. If the lead fault is intermittent, the arrhythmia may be triggered by the absence of pacing signals during the intermittent faults. However, if the fault is permanent, the intermittent arrhythmia may have some other cause which may need to he further investigated.
Indeed, without further information regarding the exact nature of the fault, the physician may not even be able to determine easily whether the fault actually exists or whether the unexpected impedance values that have been detected are caused by some malfunction in either the pacemaker or the external monitoring device. For example, the impedance detection system simply may not be calibrated properly and therefore may be generating erroneous warning signals even though there is no actual lead fault. Alternatively, the impedance detection system may not be generating warning signals at all--even though a lead fault is present--perhaps because an electrical malfunction causes the detection system to output a constant impedance value regardless of the actual impedance. This is a particularly serious problem as the physician may assume that the pacemaker is working properly even though a permanent lead fault has occurred.
As can he appreciated, it would be highly desirable to provide the physician with more complete information regarding the detected impedance values than merely a warning signal indicating that an unexpected impedance value had been detected of course, some systems may permit the physician to print out or otherwise display diagnostic information pertaining to a lead fault thereby allowing the physician to eventually come to an informed decision about the exact nature of the fault and to determine whether the fault actually exists or not. However, unless such information is provided quickly and efficiently and in a format that allows the physician to immediately determine the nature of the lead fault, it may be of little practical use to the physician, particularly in emergency situations. In general, the more difficult and time-consuming it is for the physician to access lead fault diagnostic information, the less likely he or she will routinely access that information and the less likely he or she will be able to make an informed decision regarding possible lead faults.
Accordingly, it would be desirable to provide an improved system for quickly and efficiently providing useful information to a physician regarding possible lead faults and it is to that end that aspects of the invention are primarily directed.